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Spark Plug Leak?

Discussion in 'The Garage' started by prue1, Dec 13, 2014.

  1. prue1

    prue1 XS650 Enthusiast


    I have what seems to be a exhaust leak around my left spark plug. There is a small amount of carbon soot on my two fins directly down wind of the plug. When I remove the left plug, the threads are discolored with this carbon soot the entire length of the plug. The right side spark plug threads are clean. I also have a slight "popping" sound on the left side when deaccellerating.

    Any suggestions on reducing the leak and the popping noise?

    Thank you.
  2. fredintoon

    fredintoon Fred Hill, S'toon. Top Contributor

    Hi prue1,
    even if the sparkplug thread is worn or damaged the plug's copper gasket should stop the exhaust gases leaking out.
    Try reefing the plug down tighter.
    If the thread strips out it was on it's last legs anyway.
    You can get a good salvage with a thread insert.
    And yeah, any intake or exhaust gas leak can be a source of overrun popping.
  3. retiredgentleman

    retiredgentleman XS650 Guru Top Contributor

    Use caution when tightening spark plugs. Brute force tightening may very well strip the threads. Perhaps that's the cause of your leakage now.

    My method is to use only my fingers to turn the plug fully into the head. Once fully installed, I use a torque wrench and maximum of 14 ft-lbs. If it still leaks at 14 ft-lbs then you have a real problem.......................time for a thread insert.
  4. prue1

    prue1 XS650 Enthusiast

    It has been many a year since my last "heli-coil" insert. If I need to utilize a thread insert, how is it done, where do I get one? Thanks "Great White North!
  5. prue1

    prue1 XS650 Enthusiast

    Both plugs go in easy, maybe too easy. They tighten to 14lbs. No sign that they are striped, but could they be worn out/loose, allowing the gases to creep up thru the loose threads? But shouldn't the plug washer stop the exhaust? I still get a small amount of exhaust soot escaping from the spark plug hole, at least I cannot see where else it might be coming from.
  6. NONclow

    NONclow Confirmed Laquer Breather

    Overtightening is the culprit.....or could it have been cross-threaded at some point leading to an out-of-round hole with less than stellar sealing, even with obligatory crush washer.....

    Worst case scenario, find a replacement head, and matching cover.

    HAPPY DAN XS650 Addict

    I had this problem on my bike, the spark plug seating surface was not square to the threads in the cylinder head. I used a spark plug seat cutter to get this fixed right up. I got the tool from mechanicstoolsandbits.com


    I've been wrenching for many years and have never ran into this problem. The seat cutter worked great and only took a few minutes to do.
  8. prue1

    prue1 XS650 Enthusiast

    Thanks I will look into that.

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